Platinum complexes alkynes

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

To realize milder reaction conditions, modification of the platinum catalyst system has been examined. A combined use of bis(catecholato)diboron with phosphine-free divalent platinum complex, PtCl2(cod), allows the diboration of alkynes to proceed at RT.42 The room-temperature diboration has also been achieved with a Pt(nbd)3-monophosphine (Pt/L= 1/1) catalyst.43... 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

The robustness and excellent turnover numbers of platinum complexes with terminal alkynes have made it the catalyst of choice for the synthesis of polymers and other macromolecular architechtures. Alkyne hydrosilylation with platinum has also served as a key element in the synthesis of dendrimers. Sequential reaction of an alkyne with HSiMeCl2 and lithiated phenylacetylene afforded the branching unit of a dendrimer synthesis which has been used to afford a large variety of structures at high generation.44,4411 441 ... 

These reactions can be used to prepare a novel series of complexes where cyclic alkynes can be stabilized by coordination to platinum(O).831,832 The compounds are feasible because coordination of a triple bond to platinum causes a distortion of the alkyne from linearity by displacement of the alkynic substituents back away from the platinum. Also these methods can be used to prepare platinum(O) alkyne complexes with substituents other than triphenylphosphine.833-836... 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

In 1993, Ishiyama et al.60 reported the synthesis of isomerically pure cis-1,2-bis(boryl)alkenes 53a-57a from their corresponding alkynes 53-57 (Scheme 12)60,61 via platinum complexes. The solvents did not play an important role in the reaction, but a comparison of the reaction rates at 50 °C revealed that the addition was apparently accelerated in polar... 

The properties of the complex of homoadamantyne, which is formally a cycloheptyne, more closely resemble platinum complexes of cyclohexyne than those of the seven-membered ring alkyne. The bend-back angle (50.6°) (Table VIII) is significantly larger than that of the corresponding cycloheptyne complex (41.3°) and nearly as large as that of the cyclohexyne analogue... 

T-Olefin platinum(O) complexes are important starting materials for oxidative addition see Oxidative Addition) or catalysts. Karstedt s catalysts, which are the most active ones for hydrosilylation, have been structurally characterized and found to show the structure of Pt2(M y M y )3 (9), wherein = divinyltetramethyldisiloxane." A styrene analogue Pt°(styrene)3 provides a convenient route to get an r-alkyne platinum complex by displacement (Scheme 27). DFT calculations indicate that aUcyne in the... 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

Vinyl halides add to allylic amines in the presence of Ni(cod)2 where cod=l, 5-cyclooctodine, followed by reduction with sodium borohydride. Aryl iodides add to alkynes using a platinum complex in conjunction with a palladium catalyst. A palladium catalyst has been used alone for the same purpose, and the intramolecular addition of a arene to an aUcene was accomplished with a palladium or a GaCl3 catalyst, " AUcyl iodides add intramolecularly to aUcenes with a titanium catalyst, or to alkynes using indium metal and additives. The latter cyclization of aryl iodides to alkenes was accomplished with indium and iodine or with Sml2. " ... 

Bis-silylation of alkynes is also promoted by a platinum complex use of Pt(dba)2 under CO pressure at 120°C effects bis-silylation of phenylacetylene with phenylpentamethyldisilane [52]. Nickel-catalyzed bis-silylation of diphe-nylacetylene with a benzodisilacyclobutene derivative has also been reported [53]. 

Vicente and co-workers have recently synthesized platinum(n) 7 and mixed platinum(II)/ gold(I) a-alkynyl polymers 8.53 The monomers were prepared via dehydrohalogenation between c -[PtCI2( PR jb I and various alkynes. Polymerization occurred via the reaction of the platinum complex with PPN[Au(acac)2], giving polymers with mixed metal backbones. 

Polynuclear assemblies can also be readily formed due to the ability of the alkyne ligands to form strong o-bonds with metals, as well as then-propensity for 71 coordination (t -fashion) (102) with electrophilic metal centers like Cu(I), Ag(I), and Au(I). Fornies and co-workers (44, 45, 103, 104) prepared interesting examples of di- and polynuclear alkynyl platinum complexes (Scheme 12). The [PtCu4(C=CPh)g] (105-107)... 

Another very useful route to vinylsilanes is hydrosilylation of alkynes by means of chlorosilanes, trialkoxysilanes, methyldichloro- or dialkoxysilanes, respectively, as catalyzed by ruthenium-, rhodium- or platinum complexes (equation 43a)62. A well-known example is the reaction of 31 with 7 in the presence of a ruthenium complex (equation 43b)62. 

Thermolysis of (44) in the presence of alkynes results in 1,4-disilanaphthalenes <9lOM3l73>. As already mentioned, they can be formed in higher yields during the nickel <92JOM(439)l9) and platinum catalyzed reactions <930M4987> of the (44) with diphenylalkyne and ethene respectively. Platinum-complex catalyzed dehydrogenative double silylation of alkynes, alkenes, and dienes with... 

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